Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
Organic electroluminescent devices utilize the radiative decay of excitons formed inside the emissive layer. The position of exciton formation and migration play very important role on the stability and efficiency of the devices. When holes and electrons are injected to the devices, they travel in the emissive layer, recombine, and form excitons. When the recombination zone is too narrow or close to HTL and ETL interfaces, a large buildup of charge and high concentration of excitons can occur, which can cause polaron-exciton interaction and triplet-triplet annihilation. These interactions can adversely affect the device performance, generally shortening the device lifetime. In order to increase device efficiency and improve lifetime, it is desirable to have a wider recombination zone and lower exciton concentration in the emissive layer. Therefore, the charge transporting properties of the emissive layer is important.
There are several methods for controlling the charge transporting properties of the emissive layer, such as designing compounds with the desired charge transporting properties, using a mixture of compounds with preferred transporting properties, and changing the concentration of the components in the emissive layer. Among these approaches, changing the concentration of the components in the EML to provide a gradient of materials offers a convenient way to regulate the charge transport and recombination.
Among the references that disclose gradient doping in the EML, either a hole transporting emitter such as an iridium complex or an electron transporting emitter such as a platinum complex was used. The doping concentration of the metal complex decreased or increased gradually from the anode to the cathode side. In general, the hole transporting metal complex concentration decreases away from the anode to reduce the hole transporting rate. The opposite is true for an electron transporting metal complex. Recently, Gufeng He et al. reported devices having irregular step-wise doping concentration gradients in the EML that resulted in higher efficiency than both the uniform doping and the regular gradient doping. (Phys. Status Solidi A 210, No. 3, 489-493 (2013) irregular stepwise doping in OLEDs). In Gufeng's device, bis[(4,6-difluorophenyl)-pyridinato-N,C2′](picolinate)Ir(III), FIrpic, an electron transporting metal complex, was used as the emitter. The doping concentration gradient of FIrpic was first increased and then reduces from the anode to the cathode side.
The inventors have devised novel doping concentration gradients in order to achieve further improvements in OLED device efficiency.